The production of complex integrated circuits has required the development of sophisticated methods for transferring circuit patterns to semiconductor substrates. In a typical fabrication process, an optical lithographic technique is used to transfer patterns defined on one or more masks to a photoresist-coated wafer surface. The demand to produce ever more complex circuits requires transfer of smaller and smaller circuit features. Thus, methods of increasing the resolution of lithographic systems are needed.
The basic resolution of an optical lithography system is generally related to a ratio λ/NA of an optical wavelength of the radiation used for exposure to the numerical aperture of the optical system used to direct radiation from an irradiated mask to the wafer. Thus, increases in basic resolution require decreases in irradiation wavelength or increases in optical system numerical aperture. While shorter illumination wavelengths can be used, wavelengths below about 100 nm cannot use many convenient optical materials. At some short wavelengths, suitable refractive optical materials are unavailable, and reflective optical systems are required. In some cases, these reflective optical systems must use grazing incidence reflection so that optical system design options are limited. Increases in optical system numerical aperture are generally difficult to achieve, and increased numerical aperture can reduce the tolerance of lithographic processes to defocus. As a result, increasing numerical aperture is unlikely to provide appreciable increases in basic resolution.
While increasing the basic resolution of optical lithographic systems may be impractical, the effective resolution of lithographic systems can be increased by applying one or more so-called resolution enhancement techniques (RETs). Such increases in effective resolution can be associated with a parameter k1=CD/[λ/NA]), wherein CD is a critical dimension associated with a particular set of design rules. For some circuit devices fabricated with 180 nm or smaller design rules, values of k1 less than 0.5 have been demonstrated. At k1<0.25, photolithographic exposures using a pattern of equally spaced lines of dimension equal to the critical dimension produce only an average illumination intensity at the photoresist without any appreciable intensity variation associated with pattern features. Thus, k1=0.25 can be regarded as a resolution limit for optical systems using RET.
While RETs can be used to increase effective resolution, RETs can also be used to increase process “window,” i.e. to increase process tolerance to inevitable variations in process parameters associated with masks, substrates, lithographic apparatus, and manufacturing processes. For production lithographic systems, an effective resolution of about k1=0.28 generally provides acceptable process window.
Unfortunately, the actual resolution limit achievable using most RETs depends on the pattern to be transferred as well as the placement of pattern features with respect to each other. Thus, application of a selected RET depends on the particular pattern and its placement as well as the availability of additional processing methods associated with resist development, over-exposure, multiple exposure, etch biasing, and other methods. Accordingly, improved lithographic methods, device manufacturing methods, and associated design tools are needed.